Hi all,

The problem I'm exposing is quite obvious, but unfortunatelly
can be used in a very simple way by script kiddies.

SYNOPSIS

 It's possible to slowdown (a lot) connections between two
 arbirary hosts (but at least one with the PMTU discovery enabled)
 using some spoofed TCP/IP packet. Maybe you can do more
 against some TCP/IP stack.

AFFECTED SYSTEMS

 I tried it a bit against some site, seems that at least Linux
 and some BSD are vulnerable. Anyway it is quite probable
 that almost all the TCP/IP stacks with the PMTU discovery
 enabled are vulnerable.

SOLUTION

 There isn't a clear solution.

CREDITS (!)

 me <antirez@invece.org>

DETAILS (When I talk about "the stack" I'm refering to Linux 2.4 TCP/IP stack)

 The path MTU discovery is used to optimize TCP/IP performances.
 Sorry if you don't know how it works, no flood for readers.
 Anyway the stack takes an hash table with the MTU of other ends. When
 an ICMP frag-req but DF set reaches the stack it perform a look-up
 in the hash table, searching for the old MTU, than look at the
 size of the quoted packet in the ICMP packet, and compute the new MTU
 (strong semplification). The look-up is done using even the TOS
 field, since different TOS may have different routing (I guess is for this).

 The players:

 A - some host that talks or will talk with the host B
 B - some host that talks or will talk with the host A
 C - the attacker, able to spoof IP packets

 C: sends an ICMP echo request, with some data, the source
    address set to A and the dest address set to B.
 B: creates a new entry in the hash table, if there isn't an old.
 C: sends an ICMP fragmentation needed but DF set, with the source
    address set to A and the dest address set to B,
    quoting the ICMP echo-reply response that we can guess (set the
    right TOS (usually 0x40) if you want that this works).
 B: set the new MTU in relation to the quoted packet total len.

 You may want to send this packets once every second, just to
 avoid expires. Also This may be useful if the MSS TCP option override
 the MTU (it shouldn't, but some implementations may do this),
 otherwise you can send even less spoofed packets.

 Note that shouldn't be useful to quote a packet that was really
 sent in this scenario.

EXPLOIT

 Please, write the exploit just to confirm this, don't ship
 it to lame people. I want not to release my proof-of-concepts
 code.

 That's all, can someone confirm this?

regards,
antirez